As is well known, a variety of external fixation devices are currently employed with increasing frequency in human as well as veterinary orthopaedic surgical procedures for treating complex fractures and/or fractures associated with serious damage to the cutaneous tissue.
Devices of this kind allow broken bones to be consolidated and reduced in highly critical areas of the human bony structure, especially near joints.
Customarily, the opposite ends of an external fixation device are secured to respective undamaged portions of a broken bone by means of bone screws set in the bone itself. Such is the case, for example, with a tibial fixation device having opposite ends firmly secured across the fracture to the shinbone.
In other cases, such as when the fracture affects a joint, the bone screws are set in bones adjacent to the joint. This is done, for example, with an ankle external fixation device by setting the bone screws in the tibia and the talus.
The effectiveness of such devices improves with the holding power of their bone screws. In some cases, the screws extend transversely through the bone to affect entrance and exit cortical regions.
A comprehensive literature exists in this specific technical field on research work directed to determine the critical parameters that control the holding power of bone screws.
As an example, it has been found that a relatively fine thread improves the holding power of the screw, as described in an article “Cortical profile external fixation screw maintains torque in the metaphysis”, Anatomy, Bristol, Jun. 17, 1996.
A radial preload on the screw prevents or attenuates the problem of the loss of grip or lysis in the first and/or the second cortical portion of the bone, as described in an article “Introduction and prevention of pin loosening in external fixation”, Journal of Orthopaedic Trauma, Vol. 5, No. 4, pages 485–492.
Furthermore, providing a hole in the bone prior to inserting the screw, according to the screw diameter, lowers the insertion temperature which, if excessively high, can hurt the particular bone connective tissue around the screw, as described in an article “Cancellous Bone Screw Thread Design and Holding Power”, Journal of Orthopaedic Trauma, Vol. 10, No. 7, pages 462–469.
The foregoing considerations lead to conclude that the holding power of a bone screw may be dependent on a series of features having a synergic combined effect.
However, application studies carried out at the Applicant's have resulted in the thread profile being identified as the fundamental factor of the screw holding power in the bone.
In particular, it has been found that conventional thread profiles have a common drawback in that they only provide an inferior distribution of C the stress from the force applied to penetrate the cortical portion of the bone.
In addition, conventional bone screws are of greater bulk for a given area of bone interface.
The underlying technical problem of this invention is to provide an improved bone screw having such structural and functional features as to effectively produce a self-tapping penetrative action during its insertion in the bone, and exhibiting improved holding power once in place, thereby overcoming all the drawbacks discussed hereinabove in connection with the prior art.